The present invention relates to a continuous process for preparing neral (cis-citral) in pure or enriched form by distillatively separating neral from substance mixtures comprising essentially neral and geranial (trans-citral). This distillative separation is performed in a dividing wall column or in a connection of two distillation columns in the form of a thermal coupling.
Citral (3,7-dimethylocta-2,6-dienal) is an important aroma chemical and the starting material or intermediate for preparation of a wide variety of materials of value and active ingredients. It is especially significant in this context that citral is an α,β-unsaturated aldehyde which, on the one hand, is attractive as a synthesis unit, but, on the other hand, also tends to side reactions, for example to isomerizations, owing to its high reactivity. Citral can either be isolated from natural sources or prepared synthetically, and is typically obtained in the form of mixtures of the E/Z isomers neral and geranial. For specific applications in the field of aroma or synthesis chemistry, it may be desirable to be able to use the two double bond isomers mentioned in pure or enriched form.
A problem is that the geranial and neral compounds for use in very substantially pure form differ only by the configuration of the ethylenic double bond conjugated to the particular aldehyde group and can be interconverted easily by thermal isomerization thereof. As a result, the efficient separation of geranial- and neral-containing mixtures on the industrial scale is a technical problem which still has not been solved satisfactorily.
EP 1 514 955 relates to a process for distillative workup of the electrolysis output of the electrochemical oxidation of 1,1,2,2-tetramethoxyethane with methanol to give trimethyl orthoformate in a liquid electrolyte, wherein a dividing wall column having from 30 to 150 theoretical plates is used.
DE 103 30 934 discloses a process for continuously isolating citronellal or citronellol from a crude mixture comprising at least one of these compounds by rectification. Preference is given to using those starting mixtures which are obtained by partial hydrogenation of citral and/or citronellal.
DE 102 23974 relates to a process for continuously isolating two stereoisomeric isoprenoid alcohols, specifically nerd and geraniol, from a crude mixture by rectification, the crude mixture being introduced into the side of a feed column, at least one draw column coupled to the feed column being provided and a first and a second isoprenoid alcohol being drawn off from the draw column. The feed column and the draw column are coupled such that there is no cross-mixing of vapor and condensate at least in the region of the draw of the isoprenoid alcohols.
DE 102 23 971 discloses a process for continuously isolating an α,β-unsaturated aldehyde, specifically citral from a crude mixture comprising it, by rectification. The crude mixture is introduced into the side of a feed column with a rectifying section above the feed point and a stripping section below the feed point, an upper combining column which communicates with the upper end of the rectifying section and has a condenser at the upper end of the column, and a combining column which communicates with the lower end of the stripping section and has a heater at the lower end of the column provided and a combining column which communicates with the upper combining column and the lower combining column. The α,β-unsaturated aldehyde is drawn off from the draw column as a side draw, and lower-boiling compounds are drawn off at the top of the upper combining column and higher-boiling compounds at the bottom of the lower combining column.
None of the documents cited describes the separation of stereoisomeric α,β-unsaturated aldehydes. Compared to the stereoisomeric α,β-unsaturated alcohols specified in DE 102 23 974, these have an increased reactivity, specifically easier isomerizability. DE 102 23971 likewise does not disclose a separation of stereoisomeric α,β-unsaturated aldehydes, but rather merely the separation of a mixture of cis- and trans-citral from other chemical compounds.
Proceeding from this prior art, it was an object of the present invention to provide a process for preparing very substantially pure or enriched neral (cis-citral) proceeding from inexpensively available citral. The process should be performable with a low level of apparatus complexity, in an economically viable manner and on the industrial scale, while leading especially only to a low degree of formation of decomposition products and by-products, i.e. should afford the desired product in high purity and in maximum yield.